Leaf’s central vein (6)
I believe the answer is:
midrib
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">leaf's central vein</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for midrib that I've seen before include "Large vein on leaf" , "major vein" , "part of leaf" , "Part of leaflet" , "Central leaf vein" .)